Around 300 indigenous and green activists occupied Friday the construction site of a huge hydro-electric dam across the Xingu River, a tributary of the Amazon, protest organizers said.
The demonstration at the Belo Monte dam sought to draw attention to the project at the UN Conference on Sustainable Development taking place in Rio de Janeiro, more than 3,500 kilometers (2,200 miles) to the south.
"We call on the world to let our river live," Antonia Melo, head of the Xingu River Forever Alive Movement, said in a statement.
The protesters said they dug a channel at the construction site using shovels and hoes to symbolically restore the natural flow of the river, while some used their bodies to spell out a message reading "Pare Belo Monte" (Stop Belo Monte).
The third largest dam in the world, the 11,200-megawatt scheme is one of several hydro projects billed by Brazil as providing clean energy for a fast-growing economy.
Work began a year ago, despite fierce opposition from local people and green activists.
Indigenous groups fear the dam will harm their way of life while environmentalists have warned of deforestation, greenhouse-gas emissions and irreparable damage to the ecosystem.
The Belo Monte is expected to flood an area of 500 square kilometers (200 square miles) along the Xingu and displace 16,000 people, according to the government, although some NGOs put the number at 40,000 displaced.
The UN conference, which opened Wednesday, is set to climax in a summit of an expected 116 leaders, running June 20-22.